Analytical Method Maintenance

This article provides practical tips on how to maintain test method suitability long after the formal completion of analytical method validation (AMV) studies. Case studies on how to meaningfully derive acceptance criteria for validation extensions and the validation continuum (maintenance) are described as well as an example on how to reduce analytical variability in validated systems.

Analytical Method Maintenance

There are several points to consider when running an analytical method maintenance (AMM) program. Usually, assay control results that are in established limits (e.g., plus or minus 3 s.d.), will yield valid assay results for the test samples. Whenever possible, the assay control should yield a similar assay response when compared with the test sample. Monitoring the assay control will indicate when unexpected assay result drifts or spreads may have occurred. Whenever the assay control results are close or over their established limits, there is a high probability that test sample results are also going in the same direction. This should not be ignored because it causes predictable, although not exactly measurable, errors in test results (cases 1B and 2B).1

Figure 1

Because production samples are run simultaneously with assay controls, both results should be reported and overlayed on a single statistical process control (SPC) chart (Figure 1). This will make "non-visible" data elements that lead to cases 1B and 2A-B, more "visible" and useful for information regarding the true production process performance. Had the process development, robustness, and validation studies been completed properly by using a variance component analysis matrix, the contribution to overall process variance of effect sampling (timing, number of samples, handling, storage conditions, hold times) could be estimated. If needed, sampling could be better controlled in the standard operating procedures (SOP). The likelihood of all cases (1A-B, 2A-B) occurring could then be monitored. This would also allow a much better process understanding because not only could root causes for process failures be more readily identified, but also more readily measured.

Furthermore, controlling and fixing process problems could save more batches from rejection. These steps would also be in line with the principles for quality and process analytical technology leading to faster license approvals and reduced inspections.2–5

From the robustness studies performed during analytical method development (AMD) and the intermediate precision results during AMV, the identity of the method component that may be responsible for changing test results when replaced is clarified.

This should be the main purpose of AMD and AMV reports, to not only provide results for variance components, but to also clearly identify potential bias and how they can be controlled as part of test system suitability. The validation is ideally only a confirmation of something already expected to be suitable from the AMD studies. All method components and operational limits should be sufficiently studied and properly documented in the AMD report.

This information should then be used to set meaningful limits for sample handling and storage during testing, the number of replicates, and the overall system suitability limits in the AMD and AMV report. It will otherwise be more difficult to set limits and control the implementation of new method components without this knowledge from the AMD and AMV studies. Qualifying another instrument, reference standard, critical reagent, assay control, or operator for testing may arise from the need to get test results faster. Often, use of an alternative instrument or operator is inevitable. In any case, whenever critical validated method components are exchanged, equivalency in test results between the validated method component and the alternative component should be verified.

Table 1. Instrument equivalency execution matrix

As discussed, the AMD and AMV report should indicate whatever those "critical" method components and associated risk might be involved when changing particular components. Equivalency can be accomplished similarly to AMT, provided that limits set initially still hold, by using an equivalency matrix and appropriate acceptance criteria for accuracy (matching) and (intermediate) precision.

An example for an execution matrix for instrument equivalency is illustrated in Table 1. The testing of various production lots, unlike the obligation to include this for AMTs (current regulatory expectation), should not be involved as this may not help for the purpose of the equivalency studies.1